N-methyl-D-aspartic acid receptor (hereinafter sometimes to be also indicated as NMDAR) and nicotinic acetylcholine receptor (hereinafter sometimes to be also indicated as nAChR) are known to be involved in some cognitive function processes. It is shown from animal studies that activation of NMDAR or nAChR improves some mental diseases including schizophrenia, dementia, depression, and stress vulnerability (see non-patent document 1 for NMDAR, non-patent documents 2 and 3 for nAChR).
Kynurenic acid (hereinafter sometimes to be also indicated as KYNA) is an endogenous tryptophan metabolite produced in the brain by kynurenine pathway. Tryptophan is metabolized by indoleamine 2,3-dioxygenase (IDO) and the like to produce kynurenine, and kynurenine is metabolized to produce KYNA. There are 4 kinds of known enzymes that catalyze the reaction to produce KYNA from kynurenine. That is, kynurenine-aminotransferases 1, 2, 3, and 4. Of these, KAT-II plays a key role in the production of KYNA in the brain, and it is known that KYNA concentration significantly decreases in hippocampus in KAT-II knockout mouse, as compared to that in wild-type mouse (see non-patent document 4).
KYNA is known to be an antagonist of NMDAR and nicotinic acetylcholine α7 receptor (hereinafter sometimes to be also indicated as α7nAChR). Therefore, KYNA is considered to be mainly involved in the control of presynaptic activity of GABA neuron, glutamic acid neuron via α7nAChR in the brain, and control of postsynaptic activity of glutamic acid neuron via NMDAR (see non-patent documents 5, 6 and 7).
Therefore, KAT-II inhibitor is expected to be useful for the treatment of central diseases such as schizophrenia, attention deficit/hyperactivity disorder, Alzheimer's disease, major depression and the like through activation of NMDAR and/or nAChR based on a decrease in the KYNA concentration in the brain. As documents describing the relationship between KAT-II and/or KYNA and dementia, depression, or stress vulnerability, the following are reported.
In the studies of mammals, it was confirmed that an increase in the KYNA concentration in the brain causes disorders of cognitive functions such as context learning, working memory and the like, and that an increase in the KYNA concentration may be involved in the cognitive dysfunction such as schizophrenia and the like (see non-patent documents 8-10).
R. Schwarcz et al. show that topical injection of KYNA into the brain of rodents suppresses release of dopamine, acetylcholine or glutamic acid in the site, and a possibility is proposed that attenuation of KYNA production in the brain improves cognitive function of schizophrenia (see non-patent document 11 for dopamine, non-patent document 12 for acetylcholine, non-patent document 13 for glutamic acid).
It has been reported that KYNA concentration in the cerebrospinal fluid of schizophrenia patients and bipolar disorder patients is significantly higher than that of normal volunteers and patients free of mental diseases, and the results support involvement of KYNA in the pathophysiology of schizophrenia and bipolar disorder (see non-patent document 14 for schizophrenia, and non-patent document 15 for bipolar disorder).
It has been reported that administration of a KAT-II inhibitor decreases the KYNA concentration in the brain to dialysates in a dose-dependent manner, and KAT-II inhibitors show activity in anhedonia model [chronic mild stress], which is one kind of depression models, and it has been reported that KAT-II inhibitors may be suitable for cognitive function and negative symptoms of schizophrenia (see non-patent document 16).
BTBR mouse, which is one kind of autism spectrum disorder mice, is reported to show high KYNA concentration in the medial prefrontal cortex, as compared to C57 Bl/6J mouse (see non-patent document 17).
It is known that KYNA concentration is significantly high in the putamen and caudate nucleus of postmortem brain of Alzheimer's disease patients, as compared to the control group free of dementia. It has been reported that inhibition of NMDAR by KYNA possibly causes memory disorder, learning and cognition function of Alzheimer's disease patients (see non-patent document 18).
It has been reported that patients with ischemic cerebrovascular diseases (cerebral infarction) and showing a greater kynurenine/tryptophan ratio show degraded cognitive function, and correlation between inflammatory reactions characterized by an increased IDO activity and cerebrovascular dementia is suggested (see non-patent document 19).
It has been reported that the concentration of kynurenic acid in the frontal cortex of postmortem brain of a subgroup such as HIV encephalopathy (HIV in brain) and the like in the HIV-1 (human immunodeficiency virus 1) infected patients is significantly higher than that of the control group. In addition, it is suggested that a decrease in the kynurenic acid production can be useful for an antidementia drug (see non-patent document 20).
As a compound having a KAT-II inhibitory activity, for example, the following compound has been reported.
R. Schwarcz et al. disclosed that a novel kynurenine derivative having a KAT-II inhibitory activity is effective for the treatment of cognitive impairment related to the aging of the brain and perinatal brain damage (see patent document 1).
M. M. Claffey et al. and A. B. Dounay et al. disclose that the compounds represented by the following formulas are KAT-II inhibitory compounds, and useful for the treatment of schizophrenia and cognitive deficit relating to other neurodegeneration and/or neurological disorder (see patent documents 2-4).

However, a KAT-II inhibitory action of a bicyclic or tricyclic heterocyclic compound like that of the compound of the present invention has not been reported.
Thiadiazolopyrimidone derivatives represented by the following structural formulas and the like have been sold by plural companies (e.g., AKos Consulting & Solutions GmbH, Ambinter, Aurora Fine Chemicals, ChemDiv, Inc.). However, a KAT-II inhibitory action and other pharmacological activities of these compounds have not been disclosed at all.
